Chapter 7 - Exponents and Exponential Functions - 7-3 Multipying Powers with the Same Base - Practice and Problem-Solving Exercises - Page 430: 42

$a\times a\times a^3=a^5$

$a\times a \times a^3=a^?$ To multiply powers with the same base, we add the exponents. In order to find the exponent of the product, we must find the sum of the exponents of the factors: $1+1+3=5$ We plug this exponent into the equation in place of the question mark: $a\times a\times a^3=a^5$